Universally paintable passivated galvanized steel

ABSTRACT

This invention relates to aqueous epoxy resin-containing compositions which are particularly useful for depositing coatings on freshly galvanized metals in order to protect the metal against white rust and provide a surface which is universally paintable. The aqueous compositions generally comprise (A) an oxidized alkyl ester of a dibasic carboxylic acid, (B) trivalent chromium, (C) hexavalent chromium, (D) an organic resin component consisting essentially of at least one water-dispersible or emulsifiable epoxy resin or a mixture of resins containing more than 50% by weight of at least one water-dispersible or emulsifiable epoxy resin, and (E) water, said composition being further characterized as being substantially free of strontium chromate. The wet coating on a metal substrate may be dried at a low temperature insufficient to cause curing of the epoxy resin and then be painted with any primer coating immediately or thereafter and with decorative topcoats for building materials, appliances, and automobiles. The invention also relates to metal substrates having the passivating coating on their surfaces.

This is a divisional of co-pending application Ser. No. 07/905,970 filedon Jun. 29, 1992, now U.S. Pat. No. 5,252,363.

TECHNICAL FIELD

This invention relates to an aqueous epoxy resin-containing compositionwhich is useful for coating metallic substrates in order to passivatetheir surfaces against corrosion. It relates particularly to a no bakepassivating composition contained in one package. More particularly, itrelates to galvanized steel substrates coated with the passivatingcomposition of the present invention which are universally paintableeven after time delays which normally render chromate passivatedgalvanized steel unpaintable.

BACKGROUND OF THE INVENTION

The problem with passivating coating compositions is the fact that mostpaints do not adhere well to the passivated metal surface about 48 hoursafter passivation. Steel mills engaged in galvanizing sheet metal havetraditionally considered metal which had been chrome passivated to beunpaintable after 48 hours unless the passivation coating has beenremoved by brushing. Thus, they keep two stocks of their product: oiledstock for fabricators who wish to paint the metal soon after receipt anda passivated stock for customers who do not wish to paint the metal.Although it has been discovered recently that many of the passivatedgalvanized metals that were previously thought unpaintable can actuallybe painted with a urethane-based primer such as is taught in U.S. Pat.No. 5,001,173, those passivated metals still are not universallypaintable.

In the '173 patent, my colleagues and I disclosed an aqueous epoxy resincoating composition containing chromium trioxide. It is an excellentpassivant and has had considerable commercial acceptance but thiscoating, though closer to being universally paintable than otherpassivating coatings, requires baking to achieve its passivatingproperties. One-package systems of the '173 composition are stable forthe short term but for long term storage the chromium trioxide issupplied in a solution separate from the epoxy resin component. Dibasicacid esters available from DuPont under the general trade designationDBE are taught in the '173 patent as solvents in the resinous portionsof the aqueous coating composition and in the total composition to whichthe aqueous solution of chromium trioxide has been added.

According to European Patent Application No. 0 273 408, the knownprocesses for the production of water-based corrosion resistant coatingsfor galvanized steel have the disadvantage that a large portion of thehexavalent chromium is not reduced by the organic resin which is presentin each of them. Hexavalent chromium may thus be released from thecoating on the metal, which limits the utility of the metal. Theaddition of a water soluble organic compound having a boiling pointexceeding 100° C. and at least one CH₂ OH- group and/or at least one=CHOH- group to a water-based coating agent containing hexavalentchromium, and a film-forming organic polymer prevents the release ofhexavalent chromium when in contact with aqueous solutions and preventsthe loss of corrosion resistance. The coatings are heated at from 100°to 300° C.

U.S. Pat. No. 3,185,596 teaches corrosion resistance imparting solutionsthat contain mixtures of hexavalent and trivalent chromium and watersoluble or water dispersible polyacrylic acid. Chromic acid is reducedby formaldehyde in aqueous solution. At least 5% and as much as 60% ofthe hexavalent chromium is reduced. The patent teaches a compositionwhich will yield a coating which has greatly improved corrosion andimpact resistance, flexibility, and paint bonding. It exhibits excellentperformance under vinyl paints but is not satisfactory under polyesterpaints.

A modification and extension of the teaching of the '596 patent istaught in U.S. Pat. No. 4,183,772 whereby the coating composition ismade to be universally useful under almost all types of paints. A lesseramount of the polyacrylic acid is used and a water dispensable acrylicemulsion polymer is added. The reduction of the hexavalent chromium andthe mixing of the polyacrylic acid are accomplished in generally thesame way as taught by the '596 patent but mention is made of a reactionof the polyacrylic acid with the chromium after the reduction withformaldehyde. Also, the partial reduction of hexavalent chromium ispreferably controlled so that from about 46-50% is reduced although thebroader range of 40 to 60% is also taught. Phosphoric acid is said to bean essential ingredient in the composition of the '772 patent ratherthan optional as in the '596 patent. The ratio of phosphoric acid tochromium compounds is also said in the '772 patent to be critical to thesatisfactory performance of the coating under both vinyl and polyesterpaints.

The reduction of chromic acid by formaldehyde is utilized in U.S. Pat.No. 4,170,671 wherein a water-soluble mixture of polyacrylic acid and acopolymer of acrylic acid and an acrylate is used as an emulsifier inthe polymerization of α, β-monoethylenically unsaturated monomers. Amixture of the resulting emulsion and the reduced chromium is said togive a treating liquid for the preparation of a surface so that asubsequently applied coating shows excellent adhesion.

Various types of liquid coating compositions have been applied tometallic substrates and baked thereon in order to protect the substratesagainst corrosion. Certain of such coatings are applied in conventionalmetal coil coating processes, and they must be sufficiently adherent andflexible to resist cracking, chipping and peeling. One process which hasbeen utilized for improving the corrosion resistance of various metalsubstrates generally involves the application of two coatings. The firstcoating is comprised of a material such as xanthan gum as carrier forthe other ingredients of the coating which include a chromium compoundsuch as chromium trioxide and zinc dust. On baking, the xanthan gumcontained in the coating becomes water-insoluble. Generally, bakedtemperatures of at least about 500° F. are required. Over this bakedcoating is applied a second coating which comprises a zinc rich resin.Such two-step procedures for improving the corrosion-resistance of metalare described in U.S. Pat. No. 4,026,710 (Kennedy).

U.S. Pat. No. 3,713,904 (Bernath et al.) describes compositions andmethods for producing corrosion-resistant and protective coatings onaluminum and aluminum alloys. The coating on the metal substratecomprises an organic resin, an organic solvent, an inorganic hexavalentchromium compound, an oxidizable component, phosphoric acid andstrontium chromate. On mixing, the strontium chromate and oxidizablecomponent react to reduce the hexavalent chromium to trivalent chromium.The mixture is applied to the substrate which is then heated at atemperature of from about 600° F. to about 800° F. to achieve a metaltemperature of at least about 450° F. which results in the oxidation ofa portion of trivalent chromium to hexavalent chromium resulting in astrongly adherent organic resin coating. The organic resins describedinclude epoxy resins. The patentees also describe the application ofvarious topcoats including vinyl topcoats and finish coats comprisingstrontium chromate-potassium dichromate containing fluorocarbon finishcoats.

A washcoat composition suitable for application to tinplated mild steelis described in U.S. Pat. No. 4,544,686 (Bromley et al.), and thecomposition consists of an aqueous carrier medium and a bindercomprising a thermosetting acrylic polymer, an epoxy resin, an acidcatalyst which may be phosphoric acid or chromic acid or an ammonium oramine salt thereof.

According to English language abstracts of Japanese Patent ApplicationsNos. 59052645A and 59035934A, these publications describe a weldablecoated steel sheet which has been obtained by applying a first coatingcomprising chromium trioxide, about 50% of which has been reduced to thetrivalent state, phosphoric acid, polyacrylic acid, and acrylic emulsionin water. The coated metal is then roll-coated with zinc-manganese richcoatings which contain resin such as epoxy resins. The coating then isbaked at about 260° C. for one minute.

SUMMARY OF THE INVENTION

It is an object of this invention, therefore, to provide a passivatingcoating for metal substrates which is paintable almost immediately afterbeing dried at a moderate temperature or at any later time.

It is a related object of this invention to provide an aqueous, epoxyresin-containing, metal passivant composition which need not be cured tobe universally paintable.

It is a further related object of this invention to provide a onepackage passivating composition containing water, an epoxy resin,hexavalent chromium and trivalent chromium which is stable under normalstorage conditions for an indefinitely long time.

It is another object of this invention to make it possible to eliminatethe need for keeping two stocks of galvanized steel, one stock ofpaintable but non-passivated metal and another of passivated but notuniversally paintable metal.

It is a related object to make it possible to ship galvanized steelworld-wide through many climates without concern for white rust or thepaintability of the steel.

These and other objects of this invention which will become apparentfrom the following description are achieved by an aqueous passivatingcomposition which comprises (A) an oxidized alkyl ester of a dibasiccarboxylic acid, (B) trivalent chromium, (C) hexavalent chromium, (D) anorganic resin component consisting essentially of at least onewater-dispersible or emulsifiable epoxy resin or a mixture of resinscontaining more than 50% by weight of at least one water-dispersible oremulsifiable epoxy resin, and (E) water.

These aqueous compositions are useful as pretreatment coatings on metalsurfaces, and in particular, on steel and on galvanized steel substratesThis pretreatment coating can then be dried at low temperature andcoated with weldable or non-weldable primer coatings within minutesafter being dried and anytime thereafter Decorative topcoats forbuilding materials, appliances, and automobiles may follow. Theinvention also relates to metal substrates which have been coated withthe pretreatment coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Component (A) in the aqueous compositions of the present invention isthe oxidation product of the reaction of a hexavalent chromium compoundsuch as chromium trioxide, chromic acid, or chromic anhydride with analkyl ester of a dibasic carboxylic acid for from about 1 hour to about72 hours or more at from about room temperature or less to about 195° F.The ester is oxidized to the extent that it becomes a part of thefilm-forming material when it is mixed with the resinous film-formingcomponent (D) of the aqueous passivating composition. Examples of suchesters include the dibasic esters (DBE's) mentioned above as beingsolvents in the aqueous coating compositions of the '173 patent. DBE'sare refined dimethyl esters of adipic, glutaric and succinic acids. Theyare available individually and as mixtures. For example, the productidentified as DBE has an ester content of 99.5% minimum and the esterscomprise from 10 to 25% by weight of dimethyl adipate, 55 to 75% byweight dimethyl glutarate, and 15 to 25% by weight dimethyl succinate. Atypical DBE composition is reported to comprise 17% dimethyl adipate,66% dimethyl glutarate, 16.5% dimethyl succinate and 0.2% methanol.Another dibasic ester mixture available from DuPont under thedesignation DBE-3 comprises 89% by weight dimethyl adipate, 10% byweight dimethyl glutarate, 0.5% by weight dimethyl succinate and lessthan 0.1% methanol.

The amount of oxidized ester in the aqueous compositions of thisinvention is from about 2.0% to about 10% by weight, preferably about 5%or less.

Component (B) in the aqueous compositions of the present invention isthe trivalent chromium which has been reduced from the hexavalent stateduring the oxidation of the alkyl ester of a dibasic acid. From about 10to about 90%, preferably from about 40 to about 60%, by weight of thechromium trioxide is reduced. The portion of the hexavalent chromiumthat is not reduced constitutes Component (C) of the aqueouscompositions of this invention and is available for reaction with thezinc metal on the surface of the freshly galvanized steel. Thiscomponent of the composition reacts with the epoxy resin of the coatingon the surface as well as with the zinc. The epoxy resin confers goodpaint adhesion properties to the coating even after complete conversionof the hexavalent chromium to the trivalent state by reaction with thezinc and a portion of the resin.

Component (D), the organic resin component of the aqueous compositionsof the present invention is based upon water-dispersible or emulsifiableepoxy resins. In the first embodiment, the organic resin component willcomprise a mixture of resins containing at least 50% by weight of atleast one water-dispersible or emulsifiable epoxy resin. A wide varietyof water-dispersible or emulsifiable epoxy resins can be utilized in theaqueous compositions of the present invention. Generally, the epoxyresins will have a molecular weight of from about 300 to about 100,000.More generally, the epoxy resins will have epoxide equivalent weight offrom about 150 to about 10,000, and more particularly, an epoxideequivalent weight of from about 1000 to about 3000. In one embodiment,the epoxy resin is one which typically has a weight per epoxide unit ofabout 1550-2500.

The water-dispersible or emulsifiable epoxy resins used in thisinvention may be any one of a number of well known epoxy resins whichare characterized by the presence therein of at least one epoxide group.As used in the specification and in the appended claims, epoxy resin isintended to describe the reaction products of the condensation reactionof an epihalohydrin and a hydroxy-containing compound or carboxylicacid. The epoxy resins may be of the ether or ester types although theether type epoxy resins are preferred.

Examples of ester-type epoxy resins include polyglycidyl estersobtainable by reaction of a compound containing two or more carboxylicacid groups per molecule with epichlorohydrin or glycerol dichlorohydrinin the presence of an alkali. Such polyglycidyl esters may be derivedfrom aliphatic polycarboxylic acids, e.g., succinic acid, glutaric acid,adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, ordimerised or trimerised linoleic acid; from cycloaliphaticpolycarboxylic acids such as tetrahydrophthalic acid,4-methyltetrahydrophthalic acid, hexahydrophthalic acid, and4-methylhexahydrophthalic acid; and from aromatic polycarboxylic acidssuch as phthalic acid, isophthalic acid, and terephthalic acid.

Ether-type epoxy resins are obtained by reaction of a compoundcontaining at least two free alcoholic hydroxyl and/or phenolic hydroxylgroups per molecule with an epihalohydrin under alkaline conditions, orin the alternative, in the presence of an acidic catalyst withsubsequent treatment with an alkali. The products of such reactionsinstead of being single simple compounds are generally complex mixturesof glycidyl polyethers. Generally, however, the principal product may berepresented by Formula I, as follows: ##STR1## wherein n is an integerof from zero to 30 or more, and R represents the divalent hydrocarbongroup of an aliphatic or aromatic polyhydroxy compound. These ethers maybe made from acyclic alcohols such as ethylene glycol, diethyleneglycol, and higher poly(oxyethylene) glycols, propane-1,2-diol and poly(oxypropylene) glycols, propane-1,3-diol, poly(oxytetramethylene)glycols, pentane-1,5-diol, hexane-2,4,6-triol, glycerol,1,1,1-trimethylolpropane, pentaerythritol, sorbitol, andpolyepichlorohydrins. They may also be made from cycloaliphatic alcoholssuch as resorcitol, quinitol, bis-(4-hydroxycyclohexyl)methane, and2,2-bis(4-hydroxycyclohexyl) propane, and also from alcohols havingaromatic nuclei, such as N,N-bis(2-hydroxyethyl)aniline andp,p'-bis(2-hydroxyethylamino) diphenylmethane. Or they may be made frommononuclear phenols, such as resorcinol and hydroquinone, and frompolynuclear phenols, such as bis-(4-hydroxyphenyl) methane (otherwiseknown as bisphenol F), 4,4'-dihydroxydiphenyl, bis(4-hydroxyphenyl)sulphone, 1,1,2,2-tetrakis(4-hydroxyphenyl) ethane,2,2-bis(4-hydroxyphenyl)propane, (otherwise known as bisphenol A),2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane, and novolacs formed fromaldehydes such as formaldehyde, acetaldehyde, chloral, andfurfuraldehyde, with phenol itself, and phenol substituted in the ringby chlorine atoms or by alkyl groups each containing up to 9 carbonatoms, such as 4-chlorophenol, 2 -methylphenol, and 4-tertbutylphenol.

The value of n in Formula I is determined by the relative concentrationof epichlorohydrin reactant to the polyhydroxy compound. The greater theconcentration of epichlorohydrin, the lower the value of n. In general,the value of n determines many of the characteristics of the epoxyresin. For example, the resin generally is a liquid at room temperaturesfor values of n between 0 and about 3 and solid for values of n greaterthan about 3. The physical properties of the final hardened resin alsoare determined by the value of n since, as the value of n increases, theamount of cross-linking in the resin is increased resulting in a resinof greater strength and durability.

The epoxy resins have either a mixed aliphatic aromatic or anexclusively non-benzenoid (i.e., aliphatic or cycloaliphatic) molecularstructure. The mixed aliphatic-aromatic epoxy resins generally areprepared by the well-known reaction of a bis(hydroxy-aromatic) alkane ora tetrakis-(hydroxy-aromatic) alkane with a halogen-substitutedaliphatic epoxide in the presence of a base such as, for example, sodiumhydroxide or potassium hydroxide.

In one preferred embodiment, the epoxy resins are diglycidyl ethers ofbisphenols, such as bisphenol B, F, G and H, and especially bisphenol A,which is made by reacting epichlorohydrin with bisphenol A in thepresence of an alkaline catalyst. By controlling the operatingconditions and varying the ratio of epichlorohydrin to bisphenol A,products of different molecular weight can be made.

Epoxy resins of the type described above based on various bisphenols areavailable from a wide variety of commercial sources. One group is knownby the general trade designation "Epon" resins and are available fromShell Chemical Company. For example, "Epon 820" is an epoxy resin havingan average molecular weight of about 380 and is prepared from2,2-bis-(p-hydroxyphenyl) propane and epichlorohydrin. Similarly, "Epon1031" is an epoxy resin having an average molecular weight of about 616and is prepared from epichlorohydrin and symmetricaltetrakis-(p-hydroxyphenol) ethane. "Epon 828" has a molecular weight of350- 400 and an epoxide equivalent of about 175-210. "Epon 1001" is anepoxy resin having an average molecular weight of about 1000 and anepoxide equivalent weight of 500. "Epon 1007" has an average molecularweight of about 4500 and an epoxy equivalency of about 2.0. "Epon 1009"has an epoxide equivalent of about 2400-4000.

Another group of commercially available epoxy resins is identified underthe general trade designation EPI-REZ (Celanese Resins, a division ofCelanese Coatings Company). For example, EPI-REZ 510 and EPI-REZ 509 arecommercial grades of the diglycidyl ether or bisphenol A differingslightly in viscosity and epoxide equivalent. EPI-REZ 522F is abisphenol A-epichlorohydrin resin with an epoxy equivalency of about600.

Another class of epoxy resins useful in the present invention is theepoxidized novolacs, particularly the epoxy cresol and epoxy phenolnovolacs. These are produced by reacting a novolac resin, usually formedby the reaction of orthocresol or phenol and formaldehyde withepichlorohydrin.

Epoxy resins derived from non-benzenoid materials such as aliphatic orcycloaliphatic hydroxy-containing compounds also can be utilized in thepresent invention. Epoxy resins having non-benzenoid molecularstructures generally are referred to in the art as being aliphatic epoxyresins or cycloaliphatic epoxy resins. Cycloaliphatics can be producedby the peracetic epoxidation of cyclic olefins and by the condensationof an acid such as tetrahydrophthalic with epichlorohydrin, followed bydehydrohalogenation. The aliphatic epoxy resins can be prepared byreacting hydroxy-containing aliphatic and cycloaliphatic compounds suchas aliphatic diols and triols. For example, ethylene glycol or glycerolcan be reacted with a halogen-substituted aliphatic epoxide such asepichlorohydrin (and others mentioned above) to form liquid epoxy resinscharacterized by viscosities which are lower than epoxy resins derivedfrom aromatic hydroxy compounds. When cured, such aliphatic epoxy resinsare not as brittle as the aromatic epoxy resins, and in many instances,exhibit elastomeric properties. Aliphatic epoxy resins are availablecommercially from a variety of sources including, for example, ShellChemical Company and Reichhold Chemicals, Inc. Specific examples includeEpon 562 from Shell Chemical Company having a viscosity of 90-150centipoises at about 23° C., an epoxide equivalent of 140-165, and ahydroxyl equivalent weight of about 65.

The epoxy resins will have an epoxy equivalency greater than 1.0. Byepoxy equivalence, reference is made to the average number of1,2-epoxide groups contained in the average molecule of the glycidylether or ester. As a result of the method of preparing the glycidylpolyethers and polyesters, and since they are ordinarily mixtures ofchemical compounds having somewhat different molecular weights, theepoxy equivalency of the products is not necessarily the integer 2.0.However, the equivalency is generally a value of between 1.0 and 2.0.Epoxidized novolac resins which are useful in the present inventiongenerally are prepared by the reaction of epichlorohydrin with phenolformaldehyde condensates. The epoxidized novolacs may contain more thantwo epoxy groups per molecule, and epoxidized novolacs having up to 7 tomore epoxy groups are available commercially. The use of epoxidizednovolacs containing more than two epoxy groups per molecule results inproducts containing a highly cross-linked structure.

Ultra-high molecular weight epoxy resins also may be used in theinvention. A group of such resins is available from the Shell ChemicalCompany under the general trade designation "Eponol". The ultra-highmolecular weight resins are derived from bisphenol-A and epichlorohydrinand the value of n in Formula I for Eponol Resin 53-BH-35 is about 90whereas n is about 130 for the product identified as Eponol Resin55-BH-30.

The organic resin component of the aqueous compositions of the presentinvention may comprise mixtures of epoxy resins with otherwater-dispersible or emulsifiable resins which are effective formodifying the properties of the epoxy resins and/or the coatings whichare deposited on metal substrates from the aqueous compositions. In onepreferred embodiment, the resin component comprises a mixture of anepoxy resin and at least one halogen-containing thermoplastic polymer.Halogen-containing vinyl polymers and copolymers, including vinylidenechloride homopolymers and copolymers are useful in combination with theepoxy resins. Vinylidene chloride copolymers include copolymers ofvinylidene chloride with vinyl chloride, acrylates or nitriles, thechoice of comonomers being dependent upon the properties desired.Polyvinylidene fluoride resins useful in combination with the epoxyresins of the present invention are available commercially from avariety of sources including Pennwalt Corporation. One specific exampleof a polyvinylidene fluoride available from Pennwalt is Kynar 500 resin.

The resin component of the aqueous compositions of the present inventionalso may comprise mixtures of epoxy resins with other resins capable ofmodifying the properties of the epoxy resin such as amine-formaldehyderesins, phenol-formaldehyde resins, polyamide resins, urea resins,polyolefins, polyesters, etc. as long as the additional resins do notresult in a substantial decrease of other desirable properties such asadhesion, corrosion-resistance, weldability, etc.

Among the polyesters which are useful in conjunction with epoxy resinsin the aqueous compositions of the present invention are polyesters ofaromatic dibasic acids and alkylene glycols. The polyesters also may bederived from a mixture of aromatic dicarboxylic acids containing atleast some symmetrical aromatic dicarboxylic acid, one or more acyclicdicarboxylic acids, and one or more diols. Examples of symmetricalaromatic dicarboxylic acids include terephthalic acid, bibenzoic acid,ethylene bis-p-oxy benzoic acid, tetramethylene bis-p-oxy benzoic acid,and 2,6-naphthalic acid. Other aromatic dicarboxylic acids which can beused in conjunction with the symmetrical dicarboxylic acid includeo-phthalic, isophthalic acid, etc.

The glycols which are reacted with the dibasic acids to form the desiredlinear polyesters are glycols represented by Formula II, as follows:

    HO(Y)OPH                                                   (II)

wherein Y is an alkylene group containing from about 2 to about 10carbon atoms. Examples of such glycols include ethylene glycol, 1,2- and1,3-propanediol, 1,4- butanediol, neopentyl glycol, 1,6-hexanediol,polyethylene glycol, etc.

Representative of the acyclic dicarboxylic acids which can beincorporated into the polyesters are those characterized by Formula III,as follows:

    HOOCCH.sub.2 XCH.sub.2 COOH                                (III)

wherein X is a linear divalent hydrocarbon chain composed from 2 toabout 8 atoms.

In one embodiment, mixtures of two or more acyclic dicarboxylic acidsare utilized, and the acyclic dicarboxylic acids in the mixture willdiffer from each other by at least 3 carbon atoms in the linear chain.Specific examples of the acyclic dicarboxylic acids represented by theabove Formula III include adipic acid, pimelic acid, suberic acid,azelaic acid, oxy-dibutyric acid, sebacic acid, 5-oxa-1,10-decanedioicacid, 4-n-propyl suberic acid, dodecane dioic acid, tridecane dioicacid, etc. Particularly useful combinations of aromatic and aliphaticdicarboxylic acids used in the preparation of copolyesters useful in thepresent invention include: terephthalic acid, azelaic acid andpentamethyleneglycol; terephthalic acid, isophthalic acid and adipicacid; terephthalic acid, isophthalic acid, adipic acid and sebacic acid;terephthalic acid, isophthalic acid, adipic acid and ethylene glycol;etc. Copolyesters of such mixtures can be prepared by known techniques,and they may be prepared directly from the above-identified dicarboxylicacids, or the copolyesters can be prepared from the lower alkyl estersof said dicarboxylic acids such as dimethyl terephthalate, dimethylisophthalate, dimethyl sebacate, dimethyl adibate, etc. Procedures forpreparing copolyesters useful in combination with the epoxy resins inthis invention are described in, for example, U.S. Pat. Nos. 2,623,033(Snyder) and 2,892,747 (Dye), both of which patents are herebyincorporated by reference for their disclosure of linear copolyestersderived at least in part from symmetrical aromatic dicarboxylic acids.

In one preferred embodiment, the polyesters which are utilized incombination with the epoxy resins are linear polyesters of aromaticdibasic acids and alkylene glycols. Generally, these polyesters arederived from a mixture of aromatic dibasic acids such as terephthalicand isophthalic acid with an alkylene glycol containing from 2 to about6 or 8 carbon atoms in the alkylene group. Examples of such glycolsinclude ethylene glycol, trimethylene glycol, 1,4-butylene glycol, etc.In addition to the aromatic dicarboxylic acids and the alkylene glycol,the reaction mixture also may, and preferably does contain, an acyclicdicarboxylic acid. The relative amounts of aromatic dicarboxylic acidand a cyclic dicarboxylic acid may be varied in order to obtainpolyesters having different characteristics. In general, the ratio ofequivalents of aromatic dicarboxylic acids to acyclic dicarboxylic acidwill be from about 2:1 to about 1:2 and more generally about 1:1. Theratio of dicarboxylic acid to glycol also may be varied, and the glycolis generally present in excess amounts. Thus, ratios of dicarboxylicacids to diol generally are from about 1:1 to about 1:2.

The reaction between the dicarboxylic acid mixture and the diolgenerally is effected by heating the mixture to an elevated temperaturein the presence of catalysts. Tin catalysts are especially useful forsuch purposes, and examples include dibutyl tin oxide and dibutyl tindilaurate. Other catalysts which may be utilized include antimony oxide.The polyesters and copolyesters prepared in this manner generally willhave molecular weights of from about 5000 to about 50,000 and will befurther characterized as having hydroxyl numbers of between about 5 and15.

The following are examples of polyesters which can be utilized in thecompositions of the present invention. Unless otherwise indicated in thefollowing examples, or elsewhere in the specification and claims, allparts and percentages are by weight, and all temperatures are in degreescentigrade.

POLYESTER EXAMPLE 1

Into a reaction vessel there is charged 387.6 parts (12.5 equivalents)of ethylene glycol, 228 parts (2.75 equivalents) of terephthalic acid,117.6 parts (1.42 equivalents) of isophthalic acid, 396 parts (4.2equivalents) of azelaic acid (Emerox 1144), and 0.42 part of antimonytrioxide. An inert atmosphere is maintained and the mixture is heated to240° C. with stirring while holding the vapor temperature below 125° C.A mixture of water and glycol (184 parts) is removed. When the batchtemperature reaches 240° C., the reactor is adapted for vacuumprocessing and the pressure is reduced to about 25 mm. Hg. over 1.25hours while raising the temperature to 250° C. The reaction isterminated 15 minutes after full vacuum is attained, and the vacuum isbroken with nitrogen. The molecular weight of the polyester prepared inthis manner is about 35,000, and the polyester is further characterizedas having a hydroxyl number of about 8.9 and a hydroxy-to-carboxy ratio1.06.

POLYESTER EXAMPLE 2

A reactor is charged with 17.08 parts of ethylene glycol, 35.31 parts ofneopentyl glycol, 36.81 parts of isophthalic acid, 36.83 parts ofterephthalic acid, 0.07 part of dibutyl tin octoate, and 0.13 part oftriphenyl phosphite. The mixture is heated to 230° C. using an inertsparge. The exhaust temperature is maintained at or below 110° C. as thereaction mixture is heated to 230° C. The mixture then is maintained at225°-230° C. until the acid number of the mixture is below 5. When thedesired acid number is attained, the reactor is adapted for vacuum, anda vacuum of 3 mm. Hg. or less is maintained at a temperature of about250° C. This temperature and vacuum level are maintained for a period ofabout 7.5 hours until the viscosity of the reactor contents isapproximately L (at 40% in MEK). The reaction mixture then is cooled andrecovered. The molecular weight of this polyester is about 17,500 andthe polyester is characterized as having a hydroxyl number of about 8.

Preferably, the aqueous compositions of the present invention willcomprise from about 2% to about 10% of the oxidized ester; from about0.5 to about 7% by weight of hexavalent and trivalent chromium, takentogether; from about 2 to about 25% by weight of an organic resincomponent consisting essentially of at least one water-dispersible oremulsifiable epoxy resin or a mixture of resins containing more than 50%by weight of at least one water-dispersible or emulsifiable epoxy resin;and from about 25 to about 97% by weight of water. The amount oftrivalent chromium in the aqueous composition of this invention is fromabout 0.05 to about 6.3% by weight.

The aqueous compositions of the present invention may be prepared bymixing the various components utilizing techniques well known to thoseskilled in the art. The chromic acid or other hexavalent chromiumcompound may be reacted with the dialkyl ester without the presence ofwater or other inert liquid to act as a coolant but it is preferred todissolve the chromic acid in a large quantity of water and then add theester. Suitably, the alkyl ester is added to a solution of the chromiumtrioxide in water at room temperature and the mixture allowed to standfor up to 72 hours or more, during which the exotherm of the oxidationcauses the temperature of the mixture to rise to about 195° F. Themixture of ester, water and chromium trioxide may be stirred to help thereaction. The weight ratio of water to the ester may be as low as about2:1 or 3.5:1 but it is preferred to be at least 15:1. The weight ratioof water to the chromic acid is preferably from about 6:1 to about 40:1and the weight ratio of chromic acid to the ester is preferably fromabout 4:1 to about 1:3.

The water dispersed epoxy resin is then added to the aqueous solution ofoxidized ester, unreduced chromium trioxide, and trivalent chromiumcompounds. Conventional procedures such as, for example, high speedagitation using a homo mixer or a dispersion mill, Brabender and Banburymixers can be utilized for preparing the aqueous compositions of thepresent invention.

The other ingredients of the passivating composition are suitably addedlast and mixed in similar fashion. It is highly preferable that thecomposition also contain phosphoric acid or an alkyl phosphoric acid.Examples of alkyl phosphoric acids include the lower alkyl phosphoricacids such as methyl phosphoric acid, ethyl phosphoric acid, propylphosphoric acid, and butyl phosphoric acid. Generally, when the aqueouscompositions of the present invention contain phosphoric acid or analkyl phosphoric acid, only small amounts such as, for example, fromabout 0.1 to about 3% by weight of phosphoric acid or butyl phosphoricacid are included in the aqueous compositions and this amount issufficient to provide improved properties.

The aqueous compositions of the invention also may contain small amounts(e.g., 0.1 to 5% w) of organic solvents and water-compatible or organiclubricants and colorants. In addition to being a reactant with chromiumtrioxide as discussed above, the dibasic acid esters known generally asDBE's may be used as organic solvents for the total composition.Lubricants may be included to improve polymer flow and coatingproperties. Examples of lubricants which can be included in the aqueouscompositions include glycerol esters, fatty acids, fatty acid esters,fatty acid amides, fatty acid salts, fatty alcohols, etc. Examples ofsuch lubricants include: glycerol monostearate, calcium stearate, zincstearate, silicone, polythylene waxes, polytetrafluoroethylene (PTFE),and mixtures of said waxes and PTFE.

The aqueous compositions of this invention are one package coatingcompositions which, upon drying of the wet coating composition at lowtemperatures insufficient to effect curing of the epoxy resin, providebasecoats for metallic substrates which impart corrosion resistance andare universally paintable even after long standing. The metallicsubstrates may be cold rolled and hot rolled steel, aluminized steel,and galvanized surfaces such as hot-dipped galvanized andelectro-galvanized steel, galvalume, galvaneal, etc. The aqueouscomposition of this invention is easily applied to metal sheets by acoil coating technique such as flood squeegee, direct roll, and reversecoat and by other known techniques including dipping, spraying, rollercoating, bar coating, etc. The composition is generally applied to themetal substrates in sufficient amounts to provide from 10-50 mg/ft2 of abasecoat on a dry basis although heavier or lighter coatings may beapplied. A basecoat may typically have a thickness of about 0.03 mil.After application of the aqueous coating composition to the metalsubstrate, the coating is preferably dried at a temperature sufficientto heat the metal to a temperature of from about 130° to about 300° F.although drying at room temperature is suitable. Because the aqueouscompositions of this invention are film-formers and have substantialamounts of epoxy resin therein, they can be applied to both sides of ametal coil and then dried. The coated coil is then ready to receive aprimer coat or a topcoat on each side in a subsequent coil coatingoperation. Alternatively, the basecoat coating can be used as the onlycoating on the coil. The basecoating composition can be simultaneouslyapplied to both sides of a moving coil strip.

The following example further illustrates the passivating composition ofthis invention and the method for making it. All parts are by weight.

EXAMPLE OF PASSIVATING COMPOSITION

A mixture of 12.5 parts of water, 3.7 parts of a mixture of dimethylesters of adipic, glutaric, and succinic acids (DBE, Dupont), 1.7 partsof chromium trioxide, and 0.14 part of phosphoric acid was allowed tostand in a reaction vessel for about 72 hours. The exothermic reactioncaused the temperature to rise to about 185° F. An aqueous suspension ofchromic oxide in a solution of the oxidized esters, hexavalent chromium,and phosphoric acid was obtained. To this suspension there was added 20parts of an epoxy resin sold under the trademark and number Interez 3540(EEW=2000-2500, 55% solids) and 61.5 parts of water. The resultingsolution was applied to freshly galvanized steel by a flood squeegee anda 0.03 mil thick passivating coating was obtained upon drying it at apeak metal temperature of about 150° F. The metal was painted 3 dayslater with Oxythane® urethane-based primer (Morton) and a Polyceram® 340polyester-based topcoat paint (Morton).

The adhesion and flexibility of the coatings was tested by the T-bendtest wherein a strip of the painted metal is bent back upon itself and apiece of tape is placed over the bend and pulled off to see if any loosepaint comes off with the tape. The severity of the test decreases as thenumber of thicknesses (called zero T, 1 T and so on to 5 T) of the striparound which the strip is bent increases. A strip of the metal coatedaccording to the above Example and bent over 1 T gave no tape off ofpaint.

The corrosion-resistant characteristics of the passivating compositionof this invention was determined by subjecting the metal from the aboveExample to the salt spray test of ASTM-B117. In this test, a scratch (orscribe) is made through the paint coating with a sharp instrument so asto expose the underlying metal. The scratched panel is then placed in achamber at about 95° F. where it is contacted with a spray of 5% aqueoussalt solution. After 500 hours, the maximum creep from the scribe wasless than 1/4 inch and after 1000 hours it was about 5/8 inch. There wasno creep at all from the cut edge of the panel.

While the invention has explained with reference to its embodiments, itwill be understood that various modifications will become apparent tothose skilled in the art upon reading the specification. Therefore, itshould be understood that the invention includes all modificationswithin the scope of the appended claim.

The subject matter claimed is:
 1. An aqueous passivating composition which comprises (A) an oxidized alkyl ester of a dibasic carboxylic acid, (B) trivalent chromium, (C) hexavalent chromium, (D) an organic resin component consisting essentially of at least one water-dispersible or emulsifiable epoxy resin or a mixture of resins containing more than 50% by weight of at least one water-dispersible or emulsifiable epoxy resin, and (E) water, said composition further characterized as being substantially free of strontium chromate.
 2. The composition of claim 1 wherein the oxidized ester is an oxidized dialkyl ester.
 3. The composition of claim 2 wherein the ester is an oxidized mixture of dimethyl esters.
 4. The composition of claim 1 wherein the oxidized ester is the oxidation product of the reaction between the alkyl ester and hexavalent chromium.
 5. The composition of claim 1 wherein the amount of the oxidized ester is from about 2% to about 10% by weight.
 6. The composition of claim 1 wherein the trivalent chromium is from about 0.05% to about 6.3% of the total weight. 